1. Field of the Invention
The present invention relates to a solid oxide fuel cell stack, and more particularly, to a solid oxide fuel cell stack which directly converts chemical energy of a raw material into electric energy through an electrochemical reaction.
2. Discussion of Related Art
Fuel cells are devices which directly convert chemical energy of a raw material into electric energy through an electrochemical reaction, and have some advantages in that energy efficiency thereof is high, compared with a general heat engine, and pollutants are rarely produced.
Among the fuel cells, a solid oxide fuel cell is operated at a high temperature of 600 to 1000° C., and thus may freely use a hydrocarbon or hydrogen-based fuel through internal reforming without a reformer. Further, the solid oxide fuel cell has a fuel conversion efficiency of 45 to 65%, and may have a system efficiency of 85% or more through a cogeneration system using waste heat, thereby receiving attention as a next generation eco-friendly electricity generating means.
The solid oxide fuel cell may be classified into a planar type solid oxide fuel cell and a cylindrical solid oxide fuel cell according to a type of unit cell. The cylindrical solid oxide fuel cell is advantageous in the aspect of long term stability, and the planar type solid oxide fuel cell may obtain higher power density than the cylindrical solid oxide fuel cell. Recently, a flat tubular solid oxide fuel cell in which advantages of the planar type solid oxide fuel cell and the cylindrical solid oxide fuel cell are combined has been studied.
When the solid oxide fuel cell is formed in a stacked type using a connecting material and a sealing member, a system having a capacity of a few W or a few MW or more may be realized, and thus a study on a solid oxide fuel cell stack is being widely performed.
In particular, a planar type or flat tubular solid oxide fuel cell stack has many advantages, but also has some disadvantages in that thermo-mechanical properties associated with sealing are weak, and thermal stability is low, when operated for a long period of time at a high temperature. Therefore, to overcome the disadvantages, various studies are being carried out.